The production of long metal products is generally realized in a plant by a succession of steps. Normally, in a first step, metallic scrap is provided as feeding material to a furnace which heats the scraps up to reach the liquid status. Afterwards, continuous casting equipment is used to cool and solidify the liquid metal and to form a suitably sized strand. Such a strand may then be cut to produce a suitably sized intermediate long product, typically a billet or a bloom, to create feeding stock for a rolling mill. Normally, such feeding stock is then cooled down in cooling beds. Thereafter, a rolling mill is used to transform the feeding stock, otherwise called billet or bloom depending on dimensions, to a final long product, for instance rebars or rods or coils, available in different sizes which can be used in a mechanical or construction industry. To obtain this result, the feeding stock is pre-heated to a temperature which is suitable for entering the rolling mill so as to be rolled by rolling equipment consisting of multiple stands. By rolling through these multiple stands, the feeding stock is reduced to the desired cross section and shape. The long product resulting from the former rolling process is normally cut when it is still in a hot condition; then cooled down in a cooling bed; and finally cut at a commercial length and packed to be ready for delivery to the customer.
A production plant could be ideally arranged in a way such that a direct, continuous link is established between a casting station and the rolling mill which is fed by the product of the casting procedure. In other words, the strand of intermediate product leaving the casting station would be rolled by the rolling mill continuously along one casting line. In a plant operating according to such a mode, also known as an endless mode, the continuous strand that is cast from the casting station along a corresponding casting line would be fed to the rolling mill. However, solely producing product according to such a direct charge modality does not offer the possibility of managing production interruption. Moreover, as a consequence of the normally different production rates between continuous casting apparatus and rolling mill apparatus, the production according to an exclusively endless mode is actually not preferred, or not even possible because only a part of the meltshop production would-be directly transformed into finished product.
In fact, due to the abovementioned different production rates of continuous casting apparatus and rolling mill apparatus, a plant for manufacturing long metal products is still normally arranged so that the rolling mill is fed with preliminarily cut intermediate products. Moreover, there is a desire to allow rolling of supplemental long intermediate products which may be laterally inserted into the production line directly connected to the rolling mill, for instance, by sourcing them from buffer stations which are not necessarily aligned with the rolling mill. Consequently, such feeding stock still needs to be pre-heated to a temperature which is suitable for entering the rolling mill and for being appropriately rolled therethrough.
Whatever production mode is used, in the end, to this day a huge amount of energy is commonly lost, in hot deformation processes in general and in particular in rolling by a rolling mill. This is mainly due to the fact that, during the full production route from scrap to finished products (bars, coils, rods), intermediate steps are still operationally required wherein long intermediate products, such as billets or blooms, are generated that must be cooled down to room temperature and stored, for either shorter or longer times, before the rolling phase can be actually carried out on them, according to the given overall production schedule.
Reheating from room temperature to a proper hot deformation process temperature consumes between 250 and 370 kWh/t, depending on specific process route and steel grades.
Current technologies of reheating furnaces do not allow to switch between an on and an off state of the gas fired furnace depending on actual heating requirements. Generally, only a power reduction option is given.
Due to current technologies, state of the art heating devices employed in plants for manufacturing of long metal products consume energy and generate CO2 emissions even when not required or justified from a production point of view. This amount of energy is commonly obtained from combustion of fossil fuel (heavy oil, natural gas) and thus brings about an intrinsic additional cost for companies due to the production of CO2. Given that a medium size steel production plant (1 million t of rolled product) produces around 70.000 t of CO2 per year, it is immediately clear how costs attributable to carbon footprint emissions represent a considerable burden which needs to be taken into account, on top of the costs linked to production.
In the so-called hot charging process of the prior art, billets or blooms arrive randomly, i.e. not according to a predefined energy-saving production pattern, from the continuous casting machine exit area, and thereafter for instance from a so-called hot buffer, whenever there is space available on the rolling mill. Such billets or blooms must at any rate be reheated to a temperature suitable for rolling in a dedicated fuel heating device.
As already explained, the fuel heating device can also be loaded with billets or blooms coming from a longer term storage which is effectively used as a cold buffer. In such case the fuel heating device must be continuously heated up to guarantee at any time the appropriate billets temperature for rolling operations.
None of the existing plants for production of long metal products by continuous casting and rolling processes adopts a holistic approach to reducing production costs and none of them is specifically designed to effectively take into account both throughput and energy optimization.
Analogously, none of the existing plants for production of long metal products by continuous casting and rolling processes aims at improving the eco-efficiency of manufacturing operations by adopting structured environmental management work-flows and systems based on the implementation of case-tailored but scientifically repeatable eco-efficiency strategies.
Thus, a need exists in the prior art for a method, and a corresponding system, for the production of long rolled products from casting lines which reduces the environmental impact of manufacturing operations while at the same time optimizing throughput and energy consumption, in line with the goal of sustainable development and cleaner, efficient production.